Monday, December 23, 2013

Poor Hams Scalar Analyser (PHSNA)

While working on some HF bandpass filters a few months ago I decided I needed a smaller test Instrument. I came across the PHSNA group on Yahoo groups who were using a cheap $5 AD9850 DDS module from China and an AD8307 Power detector controlled by an Arduino to make a Scalar network Analyser good to 35MHz or so. It sets the output frequency of the DDS, measures the signal with the AD8307 and sends the dBm value and frequency to a PC where the data can be captured and graphed in Excel or similar
Luckily PCB were available for the DDS carrier board and the RF detector. Thei were two versions of the DDS board. One used the widerband ERA3 amplifier, the other used a discrete 2N5109. All the components were available from Mouser. The PCBs took around 1 hour to build. To provide extra screening the Detector PCB was enclosed in a tinplate box. The following show the two units in the enclosure and the Amplifer/Interface board with the DDS PCB. Yes the PCB has been modified as the onboard filter has incorrect values. The components have been removed and an external filter fitted. The Arduino is mounted under this PCB The unit is powered from 13V supplied by the DC power connector on the Arduino, ensuring the amplifier stage has full output.

I found a W8DIZ 15m bandpass filter that I had built from the kitsandparts.com kit to try the PHSNA on. First I connected the dds output directly to the detector input and recorded the power 17 to 26MHz. I then put the filter in line and again swept 17-26MHz and recorded the power. Using excel I plotted the difference between the two sets of readings and plotthed the filter characteristic:-